PROJECT SUMMARY/ABSTRACT Cryptosporidium species, such as C. parvum, are protozoan parasites that present a significant health threat to young children and immunocompromised adults. These organisms are potential bio-terrorism agents that could incapacitate large populations, as well. Current treatment options for Cryptosporidium infections are limited. Commonly used anti-parasitic drugs are ineffective, the only FDA-approved drug is poorly efficacious, and vaccines are unavailable. Cryptosporidium species rely on inosine 5?-monophosphate dehydrogenase (IMPDH), as revealed by genomic analysis, for producing guanine nucleotides and, hence, survival. The parasite likely obtained its IMPDH gene from ?-proteobacterium, so C. parvum IMPDH (CpIMPDH) is highly diverged from the human orthologs. Consequently, selective CpIMPDH inhibitors may provide an effective strategy for the treatment of cryptosporidiosis with minimum toxicity to patients. To date, we have identified several structurally distinct classes of CpIMPDH inhibitors that have demonstrated excellent enzymatic potency, selectivity over human IMPDH, anti-parasitic activity in a cell culture model and, for one compound series, in vivo efficacy in an acute cryptosporidiosis mouse model. The overall goals of this study are to further refine CpIMPDH inhibitors to achieve efficacy in both acute and chronic mouse models of cryptosporidiosis to support the hypothesis that selective CpIMPDH inhibition is a viable treatment strategy for C. parvum infections, provide guidance for the research community with respect to optimal compound properties for in vivo efficacy and selection of pre-clinical candidates for further development. These goals will be achieved by pursuing three specific aims: 1) design and in vitro evaluation of CpIMPDH inhibitors in assays of CpIMPDH inhibitory potency and selectivity, a C. parvum cell culture infection model and ADME properties (including solubility, intestinal microsomal stability, cellular uptake, efflux, toxicity, and UGT-mediated glucuronidation); 2) assess the in vivo pharmacokinetic and acute toxicity properties of CpIMPDH inhibitors, including in vivo models of absorption and tissue distribution in both normal and C. parvum infected mice; 3) evaluate optimized CpIMPDH inhibitors in acute and chronic animal models of cryptosporidiosis assessing efficacy via imaging of fluorescent parasites, fecal oocyst excretion and gastrointestinal gross and histopathology. In addition, the impact of CpIMPDH inhibitors on commensal bacteria populations will be examined to more fully evaluate pharmacodynamics. This project will also develop a strategy for enterohepatic recycling in order to maximize gastrointestinal concentrations, while reducing systemic exposure and toxicity risk. 1